1. Field of the Invention
The present invention relates to a valve system and method for providing fluid flow between the interior and exterior of a tubular through several radial openings disposed along an axial length of the tubular. It allows opening multiple valves by one drop ball, and it may find applications in recovery of hydrocarbons from subterranean formations.
2. Prior and Related Art
A subterranean formation containing hydrocarbons consists of at least one layer of soft or fractured rock to contain the hydrocarbons, in the following called a production layer. Each production layer must be covered by a layer of impermeable rock preventing the hydrocarbons from escaping. The production layers in an oil or gas field are collectively known as the reservoir. A production well extending through the reservoir is conventionally divided into production zones, one or more production zones per production layer. A production well may extend several thousand meters vertically through the formation, and be connected to horizontal branches extending up to several kilometers through a production layer. One or more injection wells may be provided at a distance from the production well(s) in a field. The injection wells can be used to pump water, brine or gas into the formation in order to increase the pressure. Additives such as acid, solvents or surfactants may be added to the fluid in order to enhance production of hydrocarbons in processes known as “stimulating a zone”.
The production and injection wells and horizontal branches are typically lined with a steel pipe cemented to the formation and penetrated at the production layer(s). As known in the art, this pipe consists of sections having a decreasing diameter as the distance from the surface increases. In this disclosure, no distinction is made between a liner and a casing. That is, all steel pipes lining a borehole are denoted “casing” in the following. A smaller diameter production pipe or riser is provided within the casing of a production well, and used for conveying formation fluid typically consisting of a mixture of oil, water and gas, to the surface. Steel pipes of different diameters used for casing, production pipes or risers are collectively known as tubulars.
Production zones may be separated by packers sealing the annulus formed by the outer surface of a production pipe and the inner surface of a casing. The annulus formed by the borehole walls and outer circumference of a casing is usually sealed by cement and sometimes by open hole packers.
Hydraulic fracturing is a technique to enhance the flow of fluid from the formation, where the hydraulic pressure in a production zone is increased until fractures or cracks in the formation are enlarged. A slurry containing fracturing material, e.g. sand, ceramic balls or similar particles, is pumped into the cracks. When the fracturing pressure is removed, the fracturing material remains in the cracks, keeping them open. For the present invention, it is noted that the slurry containing fracturing material, e.g. sand, ceramic balls or similar particles, used for hydraulic fracturing is highly abrasive, especially where the flow of slurry changes direction from an axial direction through a pipe to a radial movement through openings in a valve. Hence, the radial openings of a valve used for hydraulic fracturing may be lined with a hard material, e.g. tungsten carbide (WC). It is further noted that pumps and other equipment for handling the abrasive slurry and pressures involved in hydraulic fracturing are readily available.
Valves are used to control the flow of formation fluid from a production zone into the production pipe through the casing, possibly through a horizontal branch. Valves are also used for controlling an injection fluid from an injection well into a zone of the formation to be stimulated. When the formation fluid from a production zone contains too much water to be economically sustainable, the production zone is shut down, typically by means of one or more valves. The valves are operated between open and closed, and possibly choked, positions using a variety if techniques, including the use of wireline tools, strings of pipes, coiled tubing, self-propagating tools known as well tractors, and drop balls. Some valves may be operated using separate hydraulic control lines. However, the space and cost required for providing separate hydraulic control lines and relatively expensive hydraulic valves quickly make hydraulically operated valves impractical for use in a tubular with many valves. This is especially true if the valves are to be opened once, and then left open.
A drop ball may be dropped or pumped into a tubular. Once it lands on a seat of a device, it prevents further fluid flow through the tubular, and hydraulic pressure builds up behind it. This hydraulic pressure may be used to operate the device, e.g. to open a valve. Thus, a drop ball may provide a simple, inexpensive and convenient way to open a valve. It does not require expensive equipment like a string of pipes, coiled tubing or a well tractor, and it can be delivered to a device in less time than it takes to make up a string, insert coiled tubing or for a well tractor to crawl to the device. However, conventional drop ball systems uses one drop ball per device, such that a drop ball passes through all bigger seats in the system until it lands on the seat intended for it. To prevent a drop ball from getting stuck on a seat not intended for it, there is a certain minimum size difference in the drop balls of the system. Hence, the number of drop balls, and consequently the number of drop ball operated devices, has been limited to about 20 per system.
In a reservoir as the one discussed above, fluid, e.g. sea water or CO2, may be pumped into a production layer from an injection well. The injected fluid ideally forces the hydrocarbons in front of it into a production pipe, e.g. a horizontal branch or a sealed off production zone of a production zone, and further through a riser to the surface. In reality, the flow of injected fluid will follow the easiest path to the nearest exit through a valve in a horizontal or vertical tubular, possibly leaving a substantial amount of desirable hydrocarbons in dead zones passed over by the easy path. Devices monitoring the formation fluid from this zone would correctly detect a break through of injected fluid. A break through of injected fluid may cause a decision to shut down the production zone, i.e. preventing more formation fluid flowing from the zone from entering the production pipe or riser, thus leaving valuable hydrocarbons in the dead zone in the formation permanently.
As discussed above, twenty or less ball operated valves would be possible along a length of tubular using a conventional drop ball system. As modern wellbores may extend more than 2000 meters vertically and/or horizontally, the average distance between ball operated valves would then exceed 100 meters, which could still constitute quite large dead zones.
U.S. patent application Ser. No. 12/705,428 “Expandable ball seat”, which is assigned to i-Tec AS and herein incorporated by reference in its entirety, discloses a system for operating multiple devices using one drop ball. This system thus may permit the use of a larger number of ball operated valves in the various wellbores or tubulars as compared to conventional systems. However, unlike other ball operated devices in a well, a valve providing fluid flow between the formation and a central bore of a tubular reduces the hydraulic pressure in the bore once it opens and fluid starts flowing from the bore through radial openings to the formation. However, this hydraulic pressure is required to open the next valve, and the pressure drop must be compensated in order to open the next valve using a drop ball. When two valves are open, an even larger pressure drop must be compensated to open the third valve, etc. In a system designed for providing a good flow of formation fluids into a tubular, the valves should preferably have large radial openings, or nozzles, such that each valve tends to contribute a correspondingly large pressure drop as it is opened. This, in turn, may limit the number of valves operable by a drop ball in a tubular.
Hence, a main objective of the present invention is to provide a tubular with a large number of valves while avoiding expensive valves, hydraulic control lines and/or expensive production time spent for operating the valves by means of wireline tools, strings, well tractors, etc. A further objective is to handle the pressure drop associated with opening a valve, thereby reducing the hydraulic pressure available for opening the next valve by means of a drop ball.